Characters and Anatomy

The anatomical feature shared by all synapsids,
and the one that gives them their name, is the 'lateral temporal
fenestra' or 'synapsid arch'. This is a large hole or arch in
the side of the skull, just behind the eye socket, which allowed
for the development of larger, longer jaw muscles. The jaw could
now be opened wider, and closed with far greater force, giving
the synapsids an advantage over their ancestors in the capture
and eating of prey. This arch has become more complex over time,
now being formed from only one bone, the others having evolved
into the bones of the mammalian inner ear.

Skull of an early synapsid, showing the position
of the synapsid arch. Also notice the carnivorous teeth that
are differently sized but not yet different in shape, but allowing
for more complex feeding mechanisms to develop.

The ancient synapsids were also amniotes, which
meant that they no longer needed to return to the water to reproduce
like their amphibian ancestors. Amniotes reproduce by means of
a 'cleidoic' or closed egg, which is protected by a semi-permeable
shell that keeps water in, but allows for exchange of gases such
as oxygen and carbon dioxide to protect the developing embryo
from the external environment, but also to keep it alive. There
is no need to lay the eggs in water, and no larval stage, so the
chances of young reaching maturity are increased. We know this
because fossil eggs, although rare, are sometimes found in association
with fossil bones, and in very rare cases special techniques can
be used to see what is inside the egg, telling us about the reproductive
cycle and the embryonic stages of the animals involved. Some descendants
of the synapsids, the placental and marsupial mammals, have evolved
to give birth to live young, an adaptation
that further increases the juvenile's chances of survival. Learn more
about
mammals.

There have also been changes in the way in
which the synapsids have moved over time. Early synapsids had
a sprawling gait, not unlike that of modern crocodiles, where
the limbs stick out to the side of the body. In this pattern
of walking the body swings from side to side, resulting
in a waddling motion. Later synapsids
adopted a 'parasagittal' gait, like that of modern mammals, with
all four feet positioned underneath the body. This gave later
synapsids increased stability when standing as the knee joints
were able to lock, the bones supporting the animal's weight. It
decreased the 'waddle', making their locomotion more efficient,
and allowing them to reach greater speeds, important both
for avoidance of predators, and capture of prey.